1. Field of the Invention
The present invention relates to a downshift control device for an automatic transmission, and more particularly to a technique for controlling a pressure applied to an engaging element through a back pressure of an accumulator in the process of shifting down of the transmission which is initiated by depressing an accelerator pedal or manually selecting a gear position while the vehicle is in a power-on condition.
2. Description of the Prior Art
There is known a shift control device for an automatic transmission as disclosed in Japanese laid-open Patent Publication No. 5-248526. In the shift control device shown in FIG. 5 of this publication, when a low clutch 50 is engaged to shift down the transmission from its 4th-speed to 3rd-speed gear position, a certain volume of oil is delivered to the low clutch 50 when conditions for start of the engagement of the low clutch 50 are satisfied, in order to eliminate a delay in time due to movement of a piston from its initial position to a position at which the piston is ready to apply a pressing force to a clutch plate. To deliver the oil to the low clutch 50, the control device is provided with an accumulator 62 having an oil chamber of a certain volume and serving to charge the clutch 50 with oil. The amount of the oil delivered to the low clutch 50 is determined so that the piston is moved to the position just before the pressing force of the piston is applied to the clutch plate. If the oil pressure in the oil chamber of the low clutch 50 is increased from this state in suitable timing, by controlling the back pressure of a low clutch accumulator 83, the low clutch 50 immediately start being engaged.
The known downshift control device, however, uses two accumulators, namely, clutch charging accumulator 62 and low clutch accumulator 83, for controlling the engaging pressure of the low clutch 50 upon downshifting from the 4th-speed to 3rd-speed gear position.